Electrolytic production of magnesium



Patented Aug. 14, 1945 ELECTROLYTIC PRODUCTION OF MAGNESIUM Cyril M.Slansky, Midland, Mich assignor to The Dow Chemical Company, Midland,-Mich., a corporation of Michigan No Drawing. Application March 2, 1942,Serial No. 433,007

6 Claims.

This invention relates to an improvement in the production of metallicmagnesium by the electrolysis of fused magnesium chloride.

In the conventional electrochemical preparation of metallic magnesium,a. fused salt bath consisting of alkali and alkaline earth metal halidesand essentially comprising magnesium chloride is electrolyzed in aferrous metal vessel, a portion of which also serves as the cathode forthe electrolysis. During the process the molten magnesium liberated atthe cathode rises to the surface of the bath and forms a pool from whichthe metal is withdrawn from time to time as it is produced.

Occasionally, however, in the electrolysis of magnesium chloride derivedfrom certain natural sources, much of the molten magnesium liberated atthe cathode fails to rise to the surface of the bath and form a pool of.metal. Instead, the electrodeposited 'metal in part detaches itself fromthe cathode in the form of discrete fine globules which bl comedispersed in the cell bath as a relatively stable emulsion and which donot coalesce to a pool of metal even after along period of time. Aselectrolysis proceeds under these conditions a. substantial part of thedispersed globules eventually reaches the anode area of the cell and isthere reoxidized. Much more of the dispersed metal together with some ofthe cell bath settles to the bottom of the vessel as a. heavy sludge,which must be withdrawn, and from which the dispersed metal cannotreadily be recovered. As a result of these phenomena in which much ofthe electrodeposited metal never becomesgcoalesced in usable form butremains dispersed in the bath and is eventually reoxidized, the currentefliciency of the magnesium-producing cell may fall to such a lowvaluethat economical production of magnesium from the particular magnesiumchloride involved is not feasible.

The occurrence of the result just described has long been acknowledgedin the published art, but the causes have remained obscure. It hasrecently been observed, however. that the dispersion ofelectrodeposite'd magnesium in the electrolysis of fused magnesiumchloride is associated, at least in large part, with the presence oftraces of inorganic boron-compound impurities, probably alkalioralkaline-earth metal borates, in the magnesium chloride. In particular,it has been found that when the magnesium chloride being electrolyzed issubstantially boron-free, the metallic magnesium liberated collectssatisfac torily as a p]. However, when the magnesium chloride being fedto the cell contains as impurity a boron-compound in a proportionequivalent to as little as 0.0015 per cent by weight of boron, theelectrodeposited magnesium tends to form as discrete globules dispersedin the cell bath, and when the boron content of the magnesium chlorideis as high as 0.03 to 0.06 per cent, the quan-' v tity of dispersionformed becomes so great, and the current eiliciency of the cell falls toso low a value, that operation becomes entirely unsatisfactory.

The formation of the undesired globular dispersion of metallic magnesiumcan, of course, be avoided by utilizing in the electrolysis onlymagnesium chloride which is substantially free of inorganic boroncompounds. However, magnesium chloride of such purity is readilyavailable from natural sources in only a, few localities. The magnesiumchloride commonly produced at most other localities contains asimpurities boron compounds in a quantity more than suflicient to causeduring electrolysis the dispersions of the metallic magnesium, and theseboron compounds often can be removed only by rather extended andexpensive chemical processing, with consequent increase in cost of themagnesium ultimately produced.

An object of the present invention, then, is to provide a method ofproducing metallic magneslum by the electrolysis of magnesium chloridecontaining as impurities traces of boron compounds wherein thedisadvantages occasioned by the boron content of the magnesium chlorideare substantially avoided. Another object is to provide a method oftreating dispersions of fine globules of metallic magnesium in moltenalkaliand alkaline-earth metal 'halide baths to recove the magnesiumcontent thereof.

According to the invention, in the electrolysis of molten salt b'athsessentially comprising magnesium chloride containing as an impurity asmall proportion of an inorganic Iboron compound, the dispersing actionof the boron compound on the electrodeposited magnesium may besubstantially eliminated by maintaining dissolved in the electrolysisbath a small proportion of manganese or a reducible inorganic manganesecompound. The presence of the manganese compound insures rapidcoalescence of the electrodeposited magnesium to a single pool, and

minimizes .to a great degree or even eliminates v altogether anydispersing of magnesium in theform of discrete fine globules in the cellbath. While no adequate theoretical explanation of the action of themanganese or manganese compound has been developed, the efiect is verymarked, and permits satisfactory production of metallic magnesium evenfrom magnesium chloride containin such quantities of boron com-- poundimpurities that operation would be otherwise virtually impossible.

-Insofar as is known, manganese or any reducible compound of manganesemay beemployed in the invention. As a practical matter, however, finelydivided metallic manganese, manganese halides, manganese oxides,especially manganese dioxide, and the alkali metal permanganates, allpreferably in the anhydrous state, are commonly used. The proportion ofmanganese or manganese compound required depends largely upon theconcentration of inorganic boron compound in the magnesium chloride, andin general should be sufllcient to provide at least one part by weightof manganese (Mn) per part of boron (B) present as boron compoundimpurity in the magnesium chloride. Optimum results are obtained whenthe manganese or manganese compound is added in a quantity such that theratio Mn:B is from 1.5 i

to 15, although other ratios are operable. It is preferable, however,that the total quantity of manganese or manganese compound addedrepresent less than about 0.1 per cent by weight of manganese relativeto the magnesiumchloride, since at higher concentrations the addedmanganese may appear in the electrodeposited magnesium to an undesireddegree. In practice, this upper limit is rarely even approached, sincethe boron content of magnesium chloride from most sources is ordinarilynot high enough to necessitate the use of such a quantity of manganeseor manganese compound.

In offsetting the undesirable eilect of boron impurities in magnesiumchloride, the manganese material may be added to the electrolysis bathin any desired manner. In general, however, best results are obtainedwhen the manganese material is introduced into the cell simultaneouslywith the boron-containing magnesium chloride, conveniently byincorporating the material in the magnesium chloride before introductionthereof into the cell.

Aside from the addition of manganese or manganese compound to the cell,or to the boron-containing magnesium chloride, the production ofmetallic magnesium according to the invention is carried out entirely inconventional manner. Bath compositions, temperatures, current densities,etc. are those used in commercial practice. Due to the presence of themanganese material the electrodeposited magnesium rapidly coalesces to asingle pool, and maybe removed from the cell from time to time as moremagnesium is liberated. The boron impurities, rendered largely harmlessby the manganese material, appear in part to escape from the cell withthe anode gases and in part to be converted to insoluble compounds,possibly also containing manganese, which settle from the cell bath andare removed at infrequent intervals.

The process of the invention is particularly advantageous in theproduction of magnesium from magnesium chloride derived from sea wateror i from natural inland .brines which contain, in addition to boroncompound impurities, a small proportion of manganese salts. In thiscase, the process for separating magnesium chloride from the sourcematerial, which is ordinarily operated to remove manganese salts, may becontrolled so that a manganese salt is present in the magnesium chlorideproduct in a proportion sufficient to minimize the adverse effect of thesmall Proper the production of magnesium by the electrolysis ofmagnesium chloride containing boron impurities in a proportioncorresponding to from 0.0015 to 0.06 per cent or more of boron. In suchinstances, it may be desirable to withdraw the dispersion from theelectrolytic cell and recover the magnesium therefrom. To this end, themolten bath containing the dispersed molten magnesium is agitated andmanganese or a reducible manganese compound is added thereto in aproportion sufllcient to effect coalescence of the molten magnesiumglobules, after which the coalesced magnesium is ladled ofl. Bestresults are obtained when the manganese material, preferably eithermetallic manganese, a manganese halide, or manganese dioxide, is addedin a proportion such that the manganese (Mn) content of the bath is atleast 1.5 times by weight the boron content thereof.

Other modes of applying the principle of the invention may be utilizedinstead of those explained, change being made as regards the detailsdisclosed, provided the steps recited in any oi. the following claims,or the equivalent thereof, are employed.

I claim:

1. In a process for the production of molten metallic magnesium whereinmagnesium chloride containing as impurity an inorganic boron-containingsubstance in a proportion corresponding to between about 0.0015 andabout 0.06 per cent of boron is electrolyzed in a fused salt bath abovethe melting point of magnesium, the method of minimizing the dispersingaction of the boron substance on the electrodeposited molten magnesiumand of insuring coalescence thereof which comprises incorporating in themagnesium chloride being fed to the electrolysis bath a substanceselected from the class consisting of manganese and reducible manganesecompounds in a proportion such that the manganese content of themagnesium chloride is at least 1.5 times by weight the boron contentthereof but does not exceed about 0.1 per cent by weight.

2. A process according to claim 1 wherein metallic manganese is added tothe magnesium chloride.

3. A process according to claim 1 wherein manganese dioxide is added-tothe magnesium chloride.

4. In a method of recovering molten magnesium from a dispersion thereofin the form of discrete fine globules in a fused salt bath above themelting point of magnesium essentially comprising magnesium chloride andcontaining as impurity an inorganic boron-containing substance in aproportion representing between about 0.0015 and about 0.06 per cent byweight of boron in the bath, the steps which comprise adding to the batha substance selected from the class consisting of manganese andreducible manganese compounds in a proportion such that the manganesecontent of the bath is at least 1.5 times by weight the boron contentthereof, agitating the bath to efl'ect coalescence of the dispersedmagnesium, and recovering the coalesced magnesium.

5. A process according to claim 4 wherein the substance addedto the bathis metallic manganese.

6. A process according to claim 4 wherein the substance added to thebath is manganese dioxide.

CYRIL M. SLANSKY.

